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Wildlife Magazine Article

"A Penny's Worth of Terror," by Randall D. Babb

"A Penny's Worth of Terror" was published in the March–April 2009 issue of Arizona Wildlife Views. To support Arizona's award-winning wildlife magazine: subscribe here.

Shrews have been misunderstood throughout history. The Greeks and Romans believed shrews to be evil. About the consequences of shrew bites to livestock, Aristotle wrote, “Shrew’s bites are dangerous, as also to other beasts of burden: Blisters develop. The bite is more dangerous if the shrew is pregnant when it bites; for then the blisters burst, while otherwise they do not.” I’m not sure why bursting blisters are worse than the non-bursting variety, as I have found both very unpleasant. Nonetheless, who wants to get all blistered up?

In Shakespeare’s day, shrews generally were regarded with dread and considered to exert “malignant influences.” The use of the word “shrew” for a nagging or spiteful person (especially a woman) and the words “shrewd” and “shrewish” are derived from the animal’s name and build upon the creature’s supposed unpleasant characteristics.

Shrews also were thought to cause paralysis in both humans and animals. Livestock that came up lame were referred to as being “shrew-struck.” Cleric and naturalist Edward Topsell wrote of shrews in 1607, “It is a ravening beast, feigning itself gentle and tame but being touched it biteth deep, and poisoneth deadly. It beareth a cruel mind, desiring to hurt anything, neither is there any creature it loveth.”

The dark cloud that shadowed the shrew’s reputation for so many centuries continues to follow it. In 1959, its already questionable name was further maligned with the release of the very bad B movie “The Killer Shrews.” Aside from the obligatory “science run amok” plotline, the film sported state-of-the-art special effects featuring dogs (disguised with papier-mache masks and mop heads on their backs) as giant, ravenous shrews. Riveting dialogue such as this exchange between the distressed heroine and the leading man, “ … There are two or three hundred giant shrews out there, monsters weighing between 50 and 100 pounds!” ... “That’s as big as a full-grown wolf!” had me rooting for the shrews before the end of the first reel. Not exactly Academy Awards material, but when was the last time you saw a movie with a shrew in it, much less a giant one?

Introduction to Arizona Shrews

There is an old proverb that says, “Three things drive a man outdoors: smoke, dripping water and a shrew.” Though this proverb undoubtedly refers to a nagging spouse, from which I have been known to flee on occasion, shrews and the quest for them have drawn rather than driven me outdoors many a time. I have a long-standing fascination with small mammals and especially these seldom-encountered and relatively little-known animals.

Shrews are the state’s smallest terrestrial mammals; depending on the species, an individual may range from 12 grams (about the weight of a 50-cent piece) to 2.5 grams (less than a penny). Thought to resemble some of the earliest mammals on our planet, shrews exhibit some primitive reproductive and skeletal characteristics. They are secretive, and though widespread, seldom are seen.

Arizona is home to seven species of shrews, most of which are found in montane forest habitats. Five belong to the long-tailed shrew group (genus Sorex) and two to the desert shrew group (genus Notiosorex). Long-tailed shrews all look very similar to one another, with dusky or dark fur; small eyes; long, flexible snouts; tails longer than half their body length; and ears largely obscured by fur. By contrast, desert shrews have shorter tails and ears that protrude well beyond their fur.

The largest of Arizona’s shrews is the water shrew. It inhabits stream banks in the White Mountains of east-central Arizona and behaves much like a miniature otter. Water shrews prey on small animals such as aquatic insect larvae, small fish, worms and beetles they capture in and near the water. The dwarf shrew is the smallest of our shrews and is found from above the tree line on the San Francisco Peaks to high-elevation meadows and woodlands on the Kaibab Plateau and in the White Mountains. Other shrews found in the state are: Merriam’s shrew, dusky shrew, Arizona shrew, Crawford’s desert shrew and Cochrum’s desert shrew. Cochrum’s desert shrew is one of the latest additions to Arizona’s fauna. Physically indistinguishable from Crawford’s desert shrew, it was described as a separate species in 2004 based on genetic analysis.

Voracious, If Not Venomous

The early belief that shrews were venomous ironically has turned out to be at least partially true. Three species of shrews (possibly more) have proven to be venomous. Only two other mammals alive today, the platypus and solenodon, share this characteristic, but only the solenodon and shrew use venom to subdue prey. Scientists have named the toxic proteins found in one species of shrew’s saliva “soricidin” and “blarina toxin.” These toxins are surprisingly potent and cause paralysis. Because it is still alive, the prey stays fresh and can be cached for later consumption. There even are a few reports of localized pain and swelling when shrews bite humans, though we are much larger than a shrew’s typical prey.

To date no species of Arizona shrews have proven venomous, but many species paralyze and cache prey through physical means. This behavior usually is seen when a shrew encounters an abundant food resource. Captive animals react this way when numerous prey items are introduced to their enclosure. They frantically run about, subduing every prey item with a decisive bite to the head. The prey is gathered up and deposited in a common cache to be consumed as needed. A single large food item often is stockpiled too, and fed upon over a protracted period of time. Some species of shrews even consume carrion.

The fierceness and voracious appetite of shrews is legendary. Though their prowess often is exaggerated, shrews are efficient and aggressive predators. The fact is that shrews are voracious by necessity. Mitochondria are the part of the cell that converts food into energy: They are the power supply for the cell and ultimately the body. Smaller mammals actually have more mitochondria per gram of tissue than do larger mammals. For a tiny shrew to keep all those mitochondria supplied with the raw material they need to make energy takes food — and lots of it.

This means that a gram of shrew tissue has a higher metabolic rate than does a gram of elephant tissue. The shrew’s elevated metabolism compensates for the enormous amount of heat shrews lose due to their small size. The greater the surface area an animal has in relation to its mass, the more rapidly it loses heat. This is one reason why your skinny aunt Mae is always cold and your fat uncle Artie is always hot. Instead of conserving heat through a larger body size, shrews must generate heat by constantly feeding a raging metabolism.

A small size comes at a great price. The shrew’s high metabolic rate does not allow it to store any appreciable amount of fat for lean times. It must remain active and search for prey throughout the year. However some species can, when food resources are scarce, go into torpor (a lowering of their body temperature and metabolic rate similar to a mini-hibernation) to conserve energy. This is like a car idling rather than remaining revved up. Gas is still being used, but at a much slower rate.

With this strategy, a desert shrew can lower its body temperature as much as 20 degrees, reducing its energy demands by as much as 80 percent. This can see the shrew through a brief tough patch, but is not a long-term solution to protracted food shortages. Most shrews consume 80–90 percent of their body weight in prey every day. This doesn’t leave much time for other pursuits. In fact, most of the shrew’s brief life, just over a year in most species, is spent looking for its next meal.

Adapted to Hardship

Desert shrews are Arizona’s most common species of shrew. Typically found at elevations below 6,000 feet, they range from oak woodland and desert grassland habitats to desertscrub habitats in Arizona. They weigh about as much as a penny and are about 2 inches long, including their short tail.

Desert shrews give birth to up to six young once or twice a year. Young weigh about .25 gram and are about 5/16 of an inch (8mm) long at birth. The eyes of a newborn are shut and the tiny creatures are hairless. A mother shrew nurses her young at first. As they mature, she regurgitates chewed food and brings prey to the nest. By 22 days of age, the young are fully furred, have their eyes open and are consuming solid food. In as little as two months, desert shrews are ready to produce their own young.

True to the reputation of their kind, desert shrews are formidable predators. One study found desert shrews preyed upon a variety of vertebrate animals, including white-footed mice, silky pocket mice, lizards and snakes. Some of these creatures weighed more than three times what the shrew did. However, vertebrate prey make up only a fraction of the desert shrew’s diet. Most of their diet is composed of arthropods such as beetles, crickets, grasshoppers, millipedes, spiders and even scorpions and tarantulas.

When attacking large or dangerous prey, desert shrews often methodically disable their victims. A desert shrew attacking a scorpion sometimes first disables or removes the stinger. With large spiders, these shrews often remove the arachnid’s legs with a series of lightning-quick rushes. But tackling large prey is a risky proposition. In one observation, the attacking shrew was bitten by a tarantula and died several hours later. In another, a desert shrew was nearly killed by a large praying mantis. Regardless, death by would-be prey seems to be a rare occurrence for desert shrews.

Life is tough for wild things and desert shrews are no exception. The demands of living in hot and arid habitats present many hurdles to wildlife. Desert shrews are equipped with a host of adaptations and behaviors that allow them to survive where no other shrew can. Moisture conservation is one of the greatest challenges for desert wildlife. Desert shrews get all the moisture they require from the food they eat. They never need to drink. They also reclaim moisture by concentrating uric acid and through special adaptations in their nose, so they lose little moisture through excretion or exhaling. Additionally, desert shrews are active mostly at night, which allows them to avoid high temperatures.

Most shrews possess powerful scent glands on their sides called “flank glands.” These are particularly large in desert shrews and are used for marking territories and supposedly to fend off predators. When a predator kills a shrew, it seldom consumes the body. It is believed shrews are just too smelly to be palatable … kind of like brussels sprouts. This “too stinky to eat” strategy does not work with birds, which for the most part possess limited olfactory capabilities. Owls are chief among the desert shrew’s predators.

There is magic in nature, especially the small, unseen things. Shrews, which defy both odds and logic, are tiny terrors that for the most part weigh little more than a penny. Next time you are compelled outdoors by a shrew, a sharp eye and a little luck will be needed if you are to glimpse Arizona’s wildlife welterweight champion.

This article was published in the March-April 2009 issue of Arizona Wildlife Views magazine. To subscribe or give a gift, order online.

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